Ultrafine manganese dioxide (MnO2) was synthesized by a micro-impinging stream reactor (MISR) built from commercial T-junctions and steel micro-capillaries. The -type MnO2 particles prepared by the MISR were of spherical morphology, approximate to 120nm in diameter and approximate to 200m(2)g(-1) in specific surface area. The particles were smaller and more uniform than those produced with traditional stirred reactors. It was also found that the morphologies and discharge specific capacitance (SC), examined by cyclic voltammograms (CV) and galvanostatic charge/discharge methods, of the prepared MnO2 depended strongly on the inlet velocity (u), volumetric flow ratio (q), and chamber configuration of the MISR. The as-prepared MnO2 under optimized conditions had a discharge specific capacitance of approximate to 211Fg(-1) and showed a capacitance decline of approximate to 18% after 1000 cycles. These values were superior to those of particles produced with stirred reactors and further confirmed the intensifying effects on micromixing behaviours of MISRs. Therefore, a MISR with good micromixing performance, short preparation time, and continuous operation will be a promising technology for the preparation of ultrafine particles.